Why These Terms Matter

Chemical and process engineering draws its vocabulary from thermodynamics, fluid mechanics, reaction kinetics, and instrumentation — and much of that vocabulary is also embedded in industry standards (ASME, API, ISA, NFPA) and regulatory language (OSHA Process Safety Management, EPA Risk Management Program). A term like "relief valve set pressure" or "turndown ratio" carries a precise technical meaning that affects equipment specification, safety case documentation, and regulatory compliance. Using these terms loosely — or misunderstanding them — leads to specification errors, failed HAZOP reviews, and in the worst case, unsafe process designs.

This glossary collects 55 of the most frequently used terms across unit operations, reaction engineering, process safety, instrumentation, and thermodynamics, organized alphabetically with plain-language explanations and standard references where applicable.

A

Activation Energy — Arrhenius equation
The minimum energy that reacting molecules must possess for a chemical reaction to proceed, expressed in the Arrhenius equation as k = A·e^(-Ea/RT). Higher activation energy makes a reaction rate more sensitive to temperature changes — this is why exothermic reactions with high activation energy are prone to thermal runaway if cooling is lost.
Adiabatic — Thermodynamics
A process in which no heat is transferred between the system and its surroundings. Adiabatic reactor temperature rise calculations are used in process safety to estimate the worst-case temperature a reactor could reach if cooling fails completely — a key input to relief system sizing.
API 650 / API 620 — American Petroleum Institute
API 650 covers welded steel storage tanks for atmospheric pressure service (up to a few psig); API 620 covers low-pressure storage tanks designed for internal pressures up to 15 psig. Tank design code selection depends on the operating pressure and determines shell thickness, roof design, and venting requirements.
Azeotrope — Vapor-liquid equilibrium
A liquid mixture that boils at a constant, unchanging composition — the vapor produced has the same composition as the liquid it boils from — making the components impossible to fully separate by simple (ordinary) distillation. Ethanol-water (95.6% ethanol) is a classic example; breaking an azeotrope requires methods like pressure-swing distillation, extractive distillation, or molecular sieves.

B

Batch Process — Process design
A manufacturing method where raw materials are processed in discrete, finite quantities ("batches") through a sequence of steps, with the equipment cleaned or reset between batches. Contrasted with continuous processing, batch operations suit low-volume, high-value products (specialty chemicals, pharmaceuticals) where recipe flexibility matters more than throughput.
BLEVE (Boiling Liquid Expanding Vapor Explosion) — Process safety
A catastrophic failure of a pressurized vessel containing a liquid held above its atmospheric boiling point (commonly LPG or other liquefied gases), typically triggered by external fire weakening the vessel shell above the liquid line. The sudden pressure release causes near-instantaneous flash vaporization and vessel fragmentation, often with an accompanying fireball. Fireproofing, pressure relief valve sizing, and fire water deluge systems are primary BLEVE mitigations.
Boiling Point Rise — Distillation / evaporation
The increase in a solution's boiling point compared to the pure solvent, caused by dissolved solutes (salts, sugars) that reduce vapor pressure. Boiling point rise must be accounted for when sizing evaporators and reboilers for concentrated solutions, or the actual heat transfer driving force will be overestimated.

C

Cavitation — Fluid mechanics / pumps
The formation and violent collapse of vapor bubbles in a liquid when local pressure drops below the fluid's vapor pressure, typically at a centrifugal pump's suction eye. Cavitation causes pitting damage to impellers, noise, vibration, and reduced pump head — prevented by ensuring available NPSH exceeds the pump's required NPSH.
CSTR (Continuous Stirred-Tank Reactor) — Reactor engineering
A reactor design in which reactants are continuously fed and product continuously withdrawn from a well-mixed vessel, so the composition inside the reactor is uniform and equal to the outlet composition. CSTRs are favored for reactions where a lower, uniform reactant concentration reduces unwanted side reactions or where temperature control of a highly exothermic reaction is critical.
Conversion (X) — Reaction engineering
The fraction of a limiting reactant that has been consumed by the reaction, defined as X = (moles reacted)/(moles fed). Conversion, together with selectivity and yield, determines reactor sizing and downstream separation loads — a reactor is sized to achieve a target conversion for the residence time and kinetics involved.
Control Valve — ISA / process control
A final control element that adjusts fluid flow by varying the size of the flow passage as directed by a controller signal, typically a pneumatically or electrically actuated valve. Control valve sizing (Cv calculation per ISA-75.01) and installed characteristic (linear, equal-percentage, quick-opening) are chosen to give the loop stable, predictable control across its operating range.

D

Dead Time — Process control
The delay between a change in a manipulated variable and the first detectable response in the measured process variable, arising from transport lag (e.g., fluid travel time through piping) or sensor lag. Dead time is the single largest limiting factor on how tightly a PID loop can be tuned — long dead time relative to the process time constant requires more conservative tuning or a dead-time compensator (Smith predictor).
DCS (Distributed Control System) — Process automation
A control architecture in which control functions are distributed across multiple networked controllers rather than concentrated in a single central computer, used to operate large continuous processes such as refineries and chemical plants. A DCS integrates regulatory control (PID loops), sequencing, alarming, and operator HMI displays across the entire facility.
Distillation — Separation processes
A separation technique that exploits differences in relative volatility between components in a liquid mixture, using repeated vaporization and condensation across a column's trays or packing to progressively concentrate the more volatile component overhead and the less volatile component in the bottoms.
DCS vs SIS — see also Instrumented Systems
The DCS handles normal process regulatory control; the Safety Instrumented System (SIS) is a functionally independent layer dedicated solely to bringing the process to a safe state on detection of a hazardous condition, per IEC 61511 / ISA-84. Keeping these systems separate is a fundamental layer-of-protection principle — a single common-cause failure should not defeat both normal control and safety shutdown.

E

Enthalpy (H) — Thermodynamics
A thermodynamic property representing the total heat content of a system at constant pressure, defined as H = U + PV. Enthalpy changes (ΔH) are used directly in energy balances around process equipment — heat exchanger duty, reactor heat of reaction, and distillation column reboiler/condenser duties are all calculated from enthalpy differences.
Equilibrium Constant (K) — Reaction engineering
A dimensionless number describing the ratio of product to reactant activities (or concentrations) at chemical equilibrium for a given reaction and temperature. A large K favors near-complete conversion to products; a small K means the reaction is limited by equilibrium regardless of residence time, requiring strategies like excess reactant, product removal, or temperature/pressure shifts (Le Chatelier's principle) to improve yield.
ESD (Emergency Shutdown) — Process safety
A predetermined, automated sequence that rapidly brings a process unit or plant to a safe state — closing isolation valves, tripping compressors, venting to flare — triggered manually or by a Safety Instrumented System on detection of a hazardous condition such as high pressure, fire, or gas detection.

F

FEED (Front-End Engineering Design) — Project engineering
The engineering phase following process design (conceptual/PFD stage) and preceding detailed design, during which enough engineering is completed to define scope, cost estimate class (typically AACE Class 2-3, ±10-20%), major equipment specifications, and a project execution plan — used as the basis for a final investment decision.
Flash Point — Process safety / NFPA
The lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near its surface. Flash point classifies flammable and combustible liquids per NFPA 30 (Class IA, IB, IC flammable; Class II, IIIA, IIIB combustible) and drives electrical area classification, tank venting, and fire protection requirements.
Flare — Process safety / relief systems
An elevated or ground-level combustion device used to safely dispose of relief valve discharges, blowdown gases, and other waste hydrocarbon streams by burning them rather than releasing them unburned to atmosphere. Flare header sizing, knockout drum design (to prevent liquid carryover), and radiant heat calculations (API 521) are core flare system design elements.

H

HAZOP (Hazard and Operability Study) — Process safety
A structured, systematic technique for identifying potential hazards and operability problems in a process by applying guide words (No, More, Less, Reverse, As Well As, Other Than) to each process parameter (flow, pressure, temperature, level) at each node of a P&ID, conducted by a multidisciplinary team. HAZOP is the most widely used process hazard analysis (PHA) method required under OSHA PSM (29 CFR 1910.119) for covered processes.
Heat Exchanger — Unit operations
Equipment designed to transfer thermal energy between two fluid streams at different temperatures without mixing them, most commonly a shell-and-tube exchanger sized per TEMA (Tubular Exchanger Manufacturers Association) standards. Heat exchanger sizing balances required duty (Q = U·A·ΔTlm) against allowable pressure drop and fouling margin.
Heat of Reaction (ΔHrxn) — Reaction engineering
The enthalpy change associated with a chemical reaction proceeding at specified conditions, negative for exothermic reactions (heat released) and positive for endothermic reactions (heat absorbed). Heat of reaction magnitude and the reaction's activation energy together determine runaway reaction risk and cooling system design requirements.
HTHA (High Temperature Hydrogen Attack) — Materials / API
A degradation mechanism in carbon and low-alloy steels exposed to hydrogen at elevated temperature and pressure, where hydrogen diffuses into the steel and reacts with carbides to form methane bubbles at grain boundaries, causing internal fissuring and loss of strength. API RP 941 (the Nelson curves) is used to select steel grades resistant to HTHA for a given temperature/pressure/hydrogen partial pressure combination.

I

IBC (Intermediate Bulk Container) — Materials handling
A reusable industrial container, typically 275-330 gallons, used for storing and transporting bulk liquids and powders; also refers to the International Building Code in a construction context — engineers should confirm which meaning applies from context.
Instrumented System — ISA / process safety
A collection of sensors, logic solvers, and final elements working together to perform a specific automated function — either the Basic Process Control System (BPCS) for normal operation or the Safety Instrumented System (SIS) for protective shutdown functions, each with its own reliability requirements defined by SIL (Safety Integrity Level) per IEC 61511.
Isentropic — Thermodynamics
A process that occurs at constant entropy, typically used as an idealized reference for compressor and turbine performance calculations. Isentropic efficiency compares actual compressor work to the theoretical work required for an ideal, reversible (isentropic) compression to the same discharge pressure.

L

LEL / UEL (Lower / Upper Explosive Limit) — Process safety
The minimum (LEL) and maximum (UEL) concentration of a flammable vapor or gas in air that can be ignited; below the LEL the mixture is too lean to burn, above the UEL it is too rich. Gas detectors are typically set to alarm at 20-25% of LEL to provide early warning well before a flammable atmosphere can form.
LOPA (Layer of Protection Analysis) — Process safety
A semi-quantitative risk assessment method, typically performed after HAZOP identifies a scenario, that tallies the independent protection layers (BPCS alarm response, relief valve, SIS, physical containment) available to reduce a hazard's frequency to a tolerable target, determining whether additional layers — often a Safety Instrumented Function at a specific SIL — are required.

M

Mass Balance — Process design
An accounting of all material entering, leaving, and accumulating within a defined process boundary, based on conservation of mass (in = out + accumulation, or in = out at steady state). Mass balances are the first calculation performed in process design, underpinning equipment sizing, yield calculations, and PFD stream tables.
MAWP (Maximum Allowable Working Pressure) — ASME pressure vessels
The maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at a designated coincident temperature, as calculated per the ASME Boiler and Pressure Vessel Code, Section VIII. Relief valve set pressure cannot exceed the vessel's MAWP under normal operating scenarios.
MOC (Management of Change) — OSHA PSM
A formal review and approval process required before implementing any change to process chemicals, technology, equipment, or procedures at a facility covered by OSHA's Process Safety Management standard, ensuring that changes do not introduce unrecognized hazards. MOC applies to both permanent and temporary changes, including "temporary" bypass jumpers that too often become permanent.

N

NPSH (Net Positive Suction Head) — Pump engineering
The difference between the suction-side pressure (converted to head) and the liquid's vapor pressure at pumping temperature, expressed as NPSH available (a system characteristic) and NPSH required (a pump characteristic from the manufacturer's curve). Cavitation is avoided by ensuring NPSH available exceeds NPSH required with adequate margin, typically 3-5 feet.

O

Overall Heat Transfer Coefficient (U) — Heat exchangers
A composite coefficient representing the combined resistance to heat transfer across a heat exchanger's tube wall, accounting for the film coefficients on both sides plus wall conduction and fouling resistance. U is central to the design equation Q = U·A·ΔTlm and typically degrades over time in service as fouling accumulates, which is why exchangers are designed with a fouling factor margin.
Overpressure Scenario — Relief systems / API 520
Any credible upset condition that could cause a vessel's pressure to exceed its MAWP — examples include fire exposure, blocked outlet, control valve failure, loss of cooling, or external fire — each of which must be evaluated to determine the governing (worst-case) required relief rate per API 520/521.

P

P&ID (Piping and Instrumentation Diagram) — Process engineering documentation
A detailed schematic showing all piping, equipment, valves, and instrumentation for a process unit, including line sizes, instrument tags, control loops, and safety devices, drawn per ISA-5.1 symbology. The P&ID is the master reference document used for construction, operation, and HAZOP review — it is more detailed than a PFD, which shows only the major process flow without piping-level detail.
PFD (Process Flow Diagram) — Process engineering documentation
A simplified schematic showing the major process equipment, primary flow paths, and key stream conditions (temperature, pressure, flow rate, composition) in a stream table, without the piping and instrumentation detail found on a P&ID. PFDs are developed early in process design to communicate the overall process concept.
PFR (Plug Flow Reactor) — Reactor engineering
A tubular reactor model that assumes fluid moves through as a series of infinitesimally thin "plugs" with no back-mixing along the flow direction, so concentration and conversion vary continuously along the reactor length. PFRs generally achieve higher conversion than a CSTR of the same volume for a given reaction because they avoid the immediate dilution to outlet concentration that occurs in a CSTR.
PSM (Process Safety Management) — OSHA 29 CFR 1910.119
A U.S. OSHA regulation requiring facilities handling threshold quantities of highly hazardous chemicals to implement fourteen management elements, including process hazard analysis, mechanical integrity, management of change, and emergency planning, aimed at preventing catastrophic releases of toxic, reactive, flammable, or explosive chemicals.
PSV (Pressure Safety/Relief Valve) — API 520/526
A spring-loaded safety device that automatically opens at a preset pressure (the set pressure) to relieve excess pressure from a vessel or system and reduces the risk of catastrophic overpressure failure, then recloses once pressure returns to a safe level. PSV sizing per API 520 Part I must account for the governing overpressure scenario's required relief rate.

Q

Quench — Reaction engineering
The rapid cooling of a reaction mixture or hot process stream, typically by direct contact with a cooler fluid, used to stop a reaction at a desired point, prevent unwanted side reactions, or protect downstream equipment from thermal damage.

R

Reboiler — Distillation
A heat exchanger at the bottom of a distillation column that vaporizes a portion of the bottoms liquid and returns it to the column, providing the upward vapor flow needed for separation. Reboiler duty is one of the largest energy consumers in a distillation-heavy chemical plant.
Reflux Ratio (R) — Distillation
The ratio of liquid returned to the top of a distillation column (reflux) to the liquid withdrawn as overhead distillate product. Higher reflux ratios improve separation (higher purity) at the cost of larger reboiler/condenser duty and column diameter — the design reflux ratio is typically set at 1.1-1.5 times the minimum reflux ratio for an economic balance.
Residence Time (τ) — Reactor engineering
The average length of time a fluid element spends inside a reactor or vessel, calculated as volume divided by volumetric flow rate (τ = V/Q) for a continuous system. Residence time, combined with reaction rate, determines the achievable conversion in a flow reactor.
Rupture Disk — Relief systems
A non-reclosing pressure relief device consisting of a thin metal membrane designed to burst at a specified pressure, providing full, unrestricted flow area instantly. Rupture disks are often installed upstream of a PSV to protect it from corrosive service or in combination for two independent layers of overpressure protection.

S

Selectivity — Reaction engineering
The fraction of converted reactant that forms the desired product rather than undesired byproducts, distinct from conversion (how much reactant reacted) and yield (moles of desired product per mole of reactant fed). A reaction can have high conversion but poor selectivity if competing side reactions consume much of the converted reactant.
SIL (Safety Integrity Level) — IEC 61511 / ISA-84
A discrete rating (SIL 1 through SIL 4) expressing the required risk reduction and corresponding reliability of a Safety Instrumented Function, with SIL 4 representing the highest reliability requirement. SIL is determined through a LOPA or similar risk assessment and drives the redundancy, testing frequency, and component selection for the safety system.
Steady State — Process design
An operating condition in which process variables (flow, temperature, pressure, composition) at any given point do not change with time, even though material and energy continuously flow through the system. Most process design calculations (mass and energy balances, equipment sizing) are performed at steady-state design conditions.

T

TEMA Class — Tubular Exchanger Manufacturers Association
A classification (Class R, C, or B) defining the mechanical design standards for shell-and-tube heat exchangers based on service severity — Class R for severe petroleum/related processing, Class C for general commercial/process applications, and Class B for chemical process service.
Turndown Ratio — Instrumentation / equipment
The ratio of an equipment's maximum to minimum controllable flow rate (or capacity) while still operating within acceptable performance limits, such as a control valve's turndown or a burner's turndown ratio. High turndown is important for equipment that must operate reliably across widely varying loads, such as during startup, turndown, and normal operation.

U

Unit Operation — Chemical engineering fundamentals
A basic, physically distinct step in a chemical process — such as distillation, absorption, extraction, filtration, drying, or heat exchange — that can be analyzed and designed using common engineering principles regardless of the specific chemicals involved. The unit operations concept, formalized in the early 20th century, is the organizing framework of chemical engineering curricula.

V

Vapor Pressure — Thermodynamics
The pressure exerted by a substance's vapor when in equilibrium with its liquid phase at a given temperature. Vapor pressure governs boiling behavior (a liquid boils when its vapor pressure equals the surrounding pressure), storage tank breathing losses, and flash calculations in distillation and separator design.
Viscosity — Fluid mechanics
A fluid property describing its resistance to shear flow — informally, its "thickness." Viscosity strongly affects pump sizing, pipe pressure drop calculations, and heat transfer film coefficients, and for many process fluids changes significantly with temperature, which must be accounted for across the full operating range.

Y

Yield — Reaction engineering
The moles (or mass) of desired product actually obtained divided by the theoretical maximum moles of product possible from the limiting reactant fed, equal to conversion multiplied by selectivity. Yield is the ultimate economic metric for a reaction step, since it determines raw material consumption and byproduct disposal costs per unit of product.